Internal combustion engines using a port fuel injection system include a throttle body assembly having a bore or passage enclosed by a butterfly-type throttle valve through which air is supplied to the plenum of the engine. With a large displacement engine, opposite faces of the throttle valve can be subjected to a substantial pressure differential when the valve is closed and the pistons are still drawing, thus causing possible deformation of the valve. Because of this, it has been the practice in larger displacement engines to utilize a pair of bores, each enclosed by a separate throttle valve, rather than employing a single, large bore with a single throttle valve.
The typical dual bore throttle valve assembly as used in the past has included a single horizontal shaft which is connected to both throttle valves. A throttle operating cable is connected to the shaft through a suitable linkage, and with this construction both valves are operated in unison. To attach the valves to the shaft, it has been common practice to mill the portions of the shaft extending across the bores to provide flat surfaces to which the valves are attached by mechanical fasteners, such as screws. Milling of the shaft tends to reduce the strength of the shaft and can cause deformation of the shaft under conditions where the valves are closed and are subjected to a substantial pressure differential.
It is also known to incorporate a progressive linkage in a dual bore throttle valve assembly for marine outboard motors. With a progressive linkage one of the valves will be opened before the other valve so that at low throttle speeds better control is obtained for docking and maneuvering of the boat.